I hadn't got a clue but apparently it is taxable and supposed to be declared by the recipient on their self assessment forms.
It is staggering that Jack is receiving nearly £15k pa for nothing. Nothing.
As others have said this is on top of her book money, Del Monte, Linda M, TV appearances, the Hellmann's ('5 figure sum' remember?), other sundry earners eg Vogue, Guardian articles. So at the very bare minimum last year she 'earned' that we know of is at the very least £25k (Hellmann's plus Patreon)